1. Field of the Invention
The technology described herein relates to non-volatile memory.
2. Description of the Related Art
Semiconductor memory has become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
When programming an EEPROM or flash memory device, such as a NAND flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in a programmed state. More information about programming can be found in U.S. Pat. No. 6,859,397 and in U.S. patent application Ser. No. 10/629,068, titled “Detecting Over Programmed Memory,” filed on Jul. 29, 2003; both applications are incorporated herein by reference in their entirety.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states (an erased state and a programmed state). Such a flash memory device is sometimes referred to as a binary flash memory device.
A multi-state flash memory device is implemented by identifying multiple distinct allowed/valid programmed threshold voltage ranges separated by forbidden ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits encoded in the memory device.
Shifts in the apparent charge stored on a floating gate can occur because of the coupling of an electric field based on the charge stored in adjacent or nearby floating gates. This floating gate to floating gate coupling phenomena is described in U.S. Pat. No. 5,867,429, which is incorporated herein by reference in its entirety. Floating gates that may cause coupling to a target floating gate may include floating gates that are on the same bit line, floating gates on the same word line, or floating gates that are across from the target floating gate because they are on both another bit line and another word line.
The floating gate to floating gate coupling phenomena occurs most pronouncedly between sets of memory cells that have been programmed at different times. For example, a first memory cell is programmed to add a level of charge to its floating gate that corresponds to one set of data. Subsequently, one or more neighboring memory cells are programmed to add a level of charge to their floating gates that correspond to a second set of data. After the one or more of the neighboring memory cells are programmed, the charge level read from the first memory cell appears to be different than programmed because of the effect of the charge on the neighboring memory cells being coupled to the first memory cell. The coupling from neighboring memory cells can shift the apparent charge level being read a sufficient amount to lead to an erroneous reading of the data stored.
The effect of the floating gate to floating gate coupling is of greater concern for multi-state devices because in multi-state devices the allowed threshold voltage ranges and the forbidden ranges are narrower than in binary devices.
As memory cells continue to shrink in size, the natural programming and erase distributions of threshold voltages are expected to increase due to short channel effects, greater oxide thickness/coupling ratio variations and more channel dopant fluctuations, reducing the available separation between adjacent states. This effect is much more significant for multi-state memories than memories using only two states (binary memories). Furthermore, the reduction of the space between word lines and of the space between bit lines will also increase the coupling between adjacent floating gates.